SAN FRANCISCO (Reuters) - The U.S. Navy can go ahead with training exercises this month using sonar off the California coast but should afterward implement new guidelines to protect whales, a U.S. appeals court said on Tuesday.

For years, U.S. courts have weighed the rights of whales to live undisturbed versus U.S. national security needs. The order of the 9th U.S. Circuit Court of Appeals was the latest effort to strike the right balance.

In August, the 9th Circuit in a split 2-1 decision put on hold a lower court injunction blocking the Navy sonar tests that wildlife supporters say harm whales. The Navy said its tests are vital to maintain military readiness.

The dissenting judge in that case asked why the Navy had dropped environmental mitigation measures to protect whales it had used from mid-2006 to January 2007.

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A new order from a different three-judge panel also cited the need to revive such mitigation efforts and asked a lower court to reconsider the issue.

It allowed the Navy to continue its exercises scheduled to end by November 22nd, and then said a previous injunction should resume pending new rules to minimize damage to whales.

"The district court shall then conduct whatever hearings it deems reasonable or necessary, and enter an appropriate preliminary injunction by January 4, 2008, the earliest approximate date at which the Navy plans its next exercise," the latest decision said.

"If the district court fails to modify the preliminary injunction so as to include mitigation measures by January 4, 2008, the Navy may make a motion before this panel for reinstatement of the stay order."